Method and apparatus for drying film by dielectric heating



DeC- 15, 1953 J. B. CUNNINGHAM ET AL 2,662,302

METHOD AND APPARATUS FOR DRYING FILM BY DIELECTRIC HEATNG Filed Feb. 7, 195o ADJ USTABLE POW! R SUPPLY l a 'IIIA lllllllllllllllllll 1111111 IN V EN TOR.5

Patented Dec. 15, 1953 METHOD AND APPARATUS FOR DRYIN- G 'EIYLMBY .DIELECTRIC .HEATING Jack B. Cunningham, North Hollywood, and .Frank'Wilburng Burbank, Calif; said Wilburn assigner `to said Cunningham- .l

applicato Eenrum v, 1959, serials@- 14am l1 `This invention relates generally to methods and apparatus for drying photographic-film, 7or other substances or articles which can take/advantage of the heating and drying conditions provided by the invention; and itrelates more particularly to a drying process and apparatus'for lm and the 'likeusing uniformhigh-intensity ultra high frequency Adielectricvfields created'by novel cavity resonator apparatus. v

In the art of dielectric'heating it is generally recognized that the amount'of heat igenerated in the load (the substance 4to be heated or dried) increases With'the frequency. We have found that ultra high frequency energy, of theorder of 90 megacycles, is especially useful, When'fproperly applied, in the heating and "drying of thin films, such as photograph-iciilm consisting of a photographic emulsion lon the usual base, e. g., cellulose acetate, cellulose nitrate, or paper. f

The proper application of this high frequency energy involves the yfactors -of high energy or field intensity, and uniformity of field.v We have found that a special type of ultra high frequency cavity resonator can be adapted for `this purpose by opening two sides thereof and making provision for continuous ltravel of the film Stock through the resonator on a continuous conveyor belt which passes through the two side openings of the resonator.

It is known in-the art dealing `:with cavity resonators that openings in the sides thereof are sources `of energy radiation, and therefore represent substantial energy dissipation-and it lisalso known that the efficiency and effectiveness of a cavity resonator .depends upon Aminimization of such energy dissipation'.

It is accordingly one majorfobject of the invention to provide a `cavity resonator'having open sides for entrance and exit of the material :torbe heated, which will nevertheless have good oscillation characteristics and sufcient .energy in the dielectric eld to'assure the,availabilityroffthe high heat that is wanted.

A further object is thefprovisionof a new and improved manner of coupling-a vhigh frequency generatorto the'reso'nator cavity. `f

The present invention, insofar as A-ilm drying is concerned, is vdirected more particularly tothe drying of the emulsion-during theoriginal 4manufacture thereof, though, of course, lit `may also -be employed in film processing as well. Inl-themanufacture of photographic'film,` the emulsion is spread by-conventional methods :in a thi-neg-layer on the usual base, which'maybejacellulosefacee claims. (ci. 34-1) -lilowsI and sinooths'ou-t' the emulsion, and functions-to distribute the silver granules more uniformly in the emulsion. This-permits the use of an `increased concentration of silverlgranules, and

hence I'results in an emulsion not Ionly of increased uniform-ityjbutof increased sensitivity.

.A- feature of the invention is that the emulsion substance, which mayv be .of any conventional type, is applied tothe iilm base'at a room temperature preferably slightly above freezing. The film -is jthen'run' through the cavity resonator in `an air conditioned room, lmaintained at a predetermined low humidity level, Iand the speed of the conveyor belt 'cnryin'g the lm through the resonator, andthe :poweron the' resonator, `are Aso regulated `as to increase the temperature `of the iilm rapidly 4fromarou-nol32" to 165 F. VThebelt speed .islpreferably about feet per minute, though vthis -is vof course subject to Wide modification. The suddenl-'yapplied :high heat, of .uniform concentration; drives oi the vmoisture and other volatile `constituentsat a Arapid rate, and thedeep, Ithrough'penetrationof the heat produced by the highfrequencyrdielectric field assures that the inside :regions lof the film will be heated `.and

dried aseffectivelyaszthe outside surfaces. .Thus VWe 'avoid the surface drying .and vfcasehardening characteristic of conventional drying processes, which, as is Well known, tend to cause' surface shrinkage, and consequent curling'and warping ess coniesout vufithjsubstlantiallyperfect' fiatneuss. It igsactually desirable,4 'invprac'tica to leave frein tvv'o percent to" fifteenV percent byy vWeight 4of jmoisture'in the nlm', since `if dried outto'a greatver degree,itwillre-absorb moisture'froin'the atmosphere ttha't extent. Accordingly, rin .practice, the power on .the resonator and -the speed Hof hlm travel lare-so regulated that the'vol'atiles rand moistureffwill be sufficientlydriven off, but with fromtwo peroenttofifteen-'percent 'of mois- 56 Y v ture allowed ltre'mainj As mentioned above, the heat and speed of illm travel are so regulated as to rapidly carry the nlm from near freezing up to 165 F. Any higher temperature has a deleterious effect on the emulsion, materially reducing its sensitivity. Accordingly, a temperature of about 165 F. being the maximum, the drying process in accordance with the invention contemplates a reduced starting temperature, so that a long temperature range will be obtained. With the emulsion applied to the film base at around freezing, it holds only a relatively small proportion of moisture at the outset. By then starting it through the drying process while continuing to hold the room atmosphere at the same reduced temperature, the emulsion can be carried through the expanded temperature range of from 32 to 165, giving a maximum opportunity to drive ofi volatiles and moisture.

The invention will be more fully understood by referring now to the following detailed description of a preferred embodiment thereof, reference for this purpose being had to the accompanying drawings, in which:

Figure l is a diagrammatic side elevational view of a film dryer in accordance with the invention;

Figure 2 is a diagrammatic section taken on line 2-2 of Figure l, showing only the cavity resonator portion of the apparatus, together with external electric circuiting therefor;

Figure 3 is a plan view of a modified cavity resonator; and

Figure 4 is a side elevational view of another alternative form of cavity resonator in accordance with the invention.

Referring ilrst to Figures l and 2, numeral I designates generally a modied re-entrant type of cavity resonator, or a medial section of a Idoughnut type of cavity resonator, having rela- Y tively close spaced condenser plates or plate portions II and I2, and inductor loops in the form of open-ended end lobes or boxes I3 and I4, preferably hollow cylinders whose interior diameter is about three times the spacing distance between plates II and I2. The lobes I3 and I4 are integrally joined to the plates II and I2, and while conveniently in the form of cylinders, they might consist of rectangular boxes, or have other forms. Also, while the lobes I3 and I4 are here shown with open ends, this is not to be regarded as essential though it is necessary that the resonator be open at each side between the plates II and I2 in order to provide for passage of the iilm. Without intention of limiting the invention, we have found that, for a lm width of 9%", the resonator I0 may be of about 48 in length, and about 12" in width, and that the plates II and I2 may be spaced 11/, and the cylinders I3 and I4 have an inside diameter of 9". Such a resonator will oscillate, when loaded, at around 90 megacycles.

A novel form of high frequency generator is employed for driving the oscillator, comprising a high frequency oscillator tube (Machlett ML 5530 whose anode 2I is seated in capacity plate 22 positioned parallel to and close spaced below resonator plate I2, and whose anode and bulb are accommodated by suitable apertures in the plates II and I2, as clearly shown in Figure 2. The grid 23 of tube 20 is connected by short lead 24 to resonator plate I I, and the two terminals of filament 25 by short leads 26 and 21 passing through small apertures 28 and 29 in plate II to reach small condenser plates 39 and 3|, respectively, positioned parallel to and about half way between the plates II and I2 on opposite sides of tube 20. These condenser plates 30 and 3 I are to be understood as arranged through any suitable means, not shown, for adjustment movement between, the plates I I and I2, as indicated by the arrows.

The letter P designates conventionally any suitable adjustable power supply unit, adapted tof supply a suitable high voltage through RF choke,- 32 to anode 2|, a suitable grid bias voltages through lead 33 and grid bias resistor 34 to lobef I4, and filament heating current through leads. 35 and 36 containing lilter chokes 31 and 38 to the terminals of filament 25.

The circuit as thus described is basically the old ultra audion circuit in which plate-to-fllament capacity is the inter-electrode plate-tolament capacity of the tube. In order to in- .crease the plate-to-filament capacity to allow suflicient grid coupling to maintain oscillations, additional capacity is introduced externally of the tube, which in this instance comprises the novel use of the adjustable condenser plates 30 and 3I placed in the electric field between the plates II and I2, and therefore coupled to the capacity plate 22 connected to the anode and placed at close spacing to the plate I2. It will be seen that the grid bias voltage is applied to the grid by way of the lead 33, lobe I4, plate II and lead 24, while suitable anode coupling and iilament-grid phasing is obtained by the adjustable condensers 30 and 3| within the electric eld of the resonator to establish oscillations at the desired frequency.

In operation, a concentrated high intensity uniform eld is established in the center section of the resonator between the plates II and I2 owing,I to the oscillatory system, the eld being at right angles to and extending between the plates I I and I2 as indicated by the dashed lines f.

Any dielectric article to be rapidly dried or heated can be placed in this field, or passed con tinuously therethrough. As here shown, an endless conveyor belt 40 is arranged to pass through the electric field between the plates II and I2 of resonatorV I0, this belt passing first over an idler roll 4I located at one side of the resonator, thence through the resonator to a motor driven roll 42, being then passed around idler roll 43 which holds it in proper engagement against the periphery of roll 42. Returning from roll 43 to roll 4I, the belt loops down and hangs below the apparatus as clearly indicated in the drawings. Any type of belt capable of withstanding the high heat within the resonator may be employed, though we prefer to use a silicone rubber belt, which is well able to stand up under the high heat conditions imposed. The iilm, designated generally by numeral 44, is carried through the resonator on this belt 4I), as clearly indicated in Figures l and 2. As shown best in Figure 2, the belt 40 and lm 44 occupy a medial plane between the resonator plates II and I2, and pass through that portion of the center section of the resonator between the condenser plate 30 and the lobe I3. As it is important to be able to regulate the speed of travel of the lm in passing through the resonator, a variable speed drive motor is employed, or alternatively, a variable speed drive may of course be employed between a constant speed motor and the drive roller.

Figure l indicates diagrammatically at 46, the outlines of an exterior housing which may be employed to enclose the resonator I0, and as will be seen, this housing is located between the infeed and takeoutr rolls 4| 'and 42, suitable apertures for the belt being of. course provided.v In the event that a refrigerated atmosphere is desired, and it is not fee'isible to refrigerate the entire room, indicated in VFigure l by the numeral 4l, merely the interior of this housing 46 need be refrigerated. Any suitable refrigerating! apparatus, not shown, may of course be employed. Also, either the entire room 41, or alternatively merely the housing 46, may beair or gas conditionedthat is tosay, maintainedl at some predetermined low humidity level. Thus, theroom 4T might' contain either air at apredetermined degree of dryness, or some inert gas, as heli-um. As. shown in Figure l,a blower 48 circulatesv and directs gas from` the conditioned room 41 through'one end of housing 4S, to be discharged towards the open space lbetween the plates Il and l2 by iiattened or fanlke nozzles 48a,v so as to direct the gas through the electric field of the resonator, and this lgas is then discharged at the 'opposite end of the housing by way of the outlet 49 Thus a continuous circulation of drying atmosphere is maintained through the housing 45, and in particular through the electric field of the .resonator where the nlm is undergoing drying, carrying away v01- atiles and moisture exaporated from the film. Any suitable means may of course be employed for these purposes, and may readily be supplied by those skilled in the art.

The return stretch of belt is indicated as passing below the housing 46. Any suitable supporting means for the housing may of course be employed, and since such features form no part of the present invention, they may be omitted from the present disclosure. Also, the resonator will be understood as mounted in any convenient or suitable fashion inside the housing 46, and While the details of such mounting means are also optional and well within the skill of the art, such details are omitted from the present disclosure. Figure 1 of the drawings does diagrammatically indicate, however, the use of insulator 50 for the support of the resonator at its lobes.

In some cases, with nlm of great width, as of the order of 42 or greater, it becomes difficult to maintain the oscillation of the resonator at the large scale which is called for, and for such leases, a resonator of the type as disclosed in Figures 1 and 2 may be made to large scale, but :severed into two individual resonators 10a and l0b which will oscillate readily by simply cutting the center section along a vertical diagonal plane, and moving the two halves a few inches apart (Figure 3). Thus, with reference to Figure 3, which is a plan View of such a resonator as has been described in connection with Figures 1 and 2, the center plate portions are formed with a diagonal gap 50, typically of a few inches of Width, and it being understood that the gap extends in a vertical diagonal plane, so that the upper and lower center plates of resonator Illa terminate along a vertical diagonal plane 6I, while the center plates of resonator lb terminate along a vertical diagonal plane 62, said planes being parallel to and spaced from one another by a matter of a few inches. In this instance, the two halves of the original resonators resulting from severing along the diagonal plane become two individual resonators, which must be individually driven. Each, therefore, has its own high frequency generator, which may be of the same type as illustrated in Figure 2. The plan view of Figure 3 accordingly shows the two high frequency generator tubes, indicated at 20a and 2Gb, and itis to be understood thateach of said tubes is mounted in 'its correspondingfresonator, and provided with circuit' connections to the resonator and toa power'-l supply u'n'it according to the method illustrated inA Figure 2.v The conveyor belt and film are shown in Figure 3 in dot-dash linesv at 34, 'and will'beiseen to pass through the center section of the resonator between the generator tubes 20a and 20o.

Each of the two resonators I'ila` and I'Db of Figure 3 will resonatevv individually, forming the same type of transverse "electric" fieldbetween their tfwo center plates. It shouldl be clear` that any ypointon the film passing' throughthe reso-l nator willv be y'outside the electric field while it passes through the gap'k 511; but that all points on the film will be withirifthe gapl region for the same' distance oftravel, andlience for the ysaine period of time, and that uniform heating is thereforeaccomplished :for thefullr Width of the film. The divided or dual resonator embodiment ofFigure 3 thus accommodates wider lengths of film than can readily be handled: by the simple embodiment of Figures 1 and 2, the .embodiment of ,Figure differing from that of Figures land 2 only in that the-center; section is divided .by a diagonal gap, and in that the two halves so formed require to be separately driven.

According to our film drying process utilizing the above described resonators, the emulsion is rst applied by conventional methods to the film base, in a room maintained at a reduced temperature, preferably just above freezing. The film is then run through the resonator, with surrounding atmospheric temperatures maintained at 32 F., or thereabouts. This can either be accomplished by reducing the room temperature, or reducing merely the temperature of the atmosphere within the resonator housing. In any case, the film at a temperature of around 32 F. is, after application of the emulsion, run at the reduced temperature into the resonator. The speed of travel of the lm through the resonator, and the power on the resonator, are so controlled and correlated that the film is raised to a temperature approximating F., or at least not substantially in excess of that temperature, by the time the nlm leaves the resonator. This treatment rapidly drives off the volatiles, reduces the moisture to the necessary degree, further distributes the silver grains in the emulsion, and uniformly dries the lm base as well as the emulsion layer so that the usual curl and warping does not occur.

A flatter lm product, of greater uniformity and` improved sensitivity, is obtained by this process. The process has one very important result, in that the application of the high frequency electric energy is found to thoroughly sterilize the lm, effectively and completely killing all fungi in the emulsion. It is a known fact that fungi in the emulsion attacks and destroys the emulsion, particularly in tropical climates. The present drying process eliminates this source of lm deterioration.

Figure 4 is a further alternative embodiment, wherein the resonator 10c has the usual lobes 13o and |40, but wherein the center section is made up of plates llc and I 2c which are curved to accommodate a curved article to be heated. In this instance, the illustrative articles to be heated and dried are plastic coat hangers carried by a curved conveyor belt 40e, and guided at the ends by ceramic guides 10. Aside from these innovations to adapt the device to a curved article to be heated and dried, the resonator may conform to the teachings of Figures 1 and 2, and no further discussion is believed necessary.

. We have now disclosed the invention in a typical preferred form, but it will be understood that many variations and modifications are possible, and the invention isaccordingly to be limited only in accordance with a fair interpretation of the following claims.

We claim:

1. The method of 111m drying that includes: initially reducing the illm to be dried to substantially a temperature of 32 F., and continuously passing said lm through the electric eld of a high frequency cavity resonator at a rate of speed to increase the temperature of said film while passing through said resonator to a nal value which is substantially in excess of normal atmospheric temperature but below the temperature at which the film is adversely affected.

2. The method of lm drying that includes: initially reducing the illm to be dried to substantially a temperature of 32 F., continuously passing said film through the electric eld of a high frequency cavity resonator at a. rate of speed to increase the temperature of said lm while passing through said resonator to a final value substantially in excess of normal atmospheric temperature but below the temperature at which the fllm is adversely affected, and passing a gas of predetermined low humidity through said high frequency electric field while said lm is passing therethrough.

JACK B, CUNNINGHAM.

FRANK WILBURN.

References Cited in the file Of this patent UNITED STATES PATENTS 

